Newspaper article covering unknown material (7)
I believe the answer is:
organza
'material' is the definition.
(I know that organza is a type of material)
'newspaper article covering unknown' is the wordplay.
'newspaper' becomes 'organ' (organ can refer to a newspaper or journal).
'article' becomes 'a' (the indefinite article in English).
'covering' is an insertion indicator.
'unknown' becomes 'z' (algebra).
'organ'+'a'='organa'
'organa' going around 'z' is 'ORGANZA'.
(Other definitions for organza that I've seen before include "Stuff" , "Transparent silk material" , "Silky cloth" , "Thin material" , "Transparent silky material" .)